Fold SkJSON into Skia/utils

It's a tiny, core-ish component -- might as well treat as such to
simplify dependencies.

Change-Id: I6f31ce2d151f9a629d88bfc7f15d64891d5150c0
Reviewed-on: https://skia-review.googlesource.com/135780
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>

1 files changed, 360 insertions, 0 deletions

diff --git a/src/utils/SkJSON.h b/src/utils/SkJSON.h
new file mode 100644
index 0000000000..8baf9fb8fc
--- /dev/null
+++ b/src/utils/SkJSON.h
@@ -0,0 +1,360 @@
+/*
+ * Copyright 2018 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkJSON_DEFINED
+#define SkJSON_DEFINED
+
+#include "SkArenaAlloc.h"
+#include "SkTo.h"
+#include "SkTypes.h"
+
+#include <cstring>
+
+class SkString;
+class SkWStream;
+
+namespace skjson {
+
+/**
+ *  A fast and likely non-conforming JSON parser.
+ *
+ *  Some known limitations/compromises:
+ *
+ *    -- single-precision FP numbers
+ *
+ *    -- missing string unescaping (no current users, could be easily added)
+ *
+ *
+ *  Values are opaque, fixed-size (64 bits), immutable records.
+ *
+ *  They can be converted to facade types for type-specific functionality.
+ *
+ *  E.g.:
+ *
+ *     if (v.is<ArrayValue>()) {
+ *         for (const auto& item : v.as<ArrayValue>()) {
+ *             if (const NumberValue* n = item) {
+ *                 printf("Found number: %f", **n);
+ *             }
+ *         }
+ *     }
+ *
+ *     if (v.is<ObjectValue>()) {
+ *         const StringValue* id = v.as<ObjectValue>()["id"];
+ *         if (id) {
+ *             printf("Found object ID: %s", id->begin());
+ *         } else {
+ *             printf("Missing object ID");
+ *         }
+ *     }
+ */
+class alignas(8) Value {
+public:
+    enum class Type {
+        kNull,
+        kBool,
+        kNumber,
+        kString,
+        kArray,
+        kObject,
+    };
+
+    /**
+     * @return    The type of this value.
+     */
+    Type getType() const;
+
+    /**
+     * @return    True if the record matches the facade type T.
+     */
+    template <typename T>
+    bool is() const { return this->getType() == T::kType; }
+
+    /**
+     * Unguarded conversion to facade types.
+     *
+     * @return    The record cast as facade type T&.
+     */
+    template <typename T>
+    const T& as() const {
+        SkASSERT(this->is<T>());
+        return *reinterpret_cast<const T*>(this);
+    }
+
+    /**
+     * Guarded conversion to facade types.
+     *
+     * @return    The record cast as facade type T*.
+     */
+    template <typename T>
+    operator const T*() const {
+        return this->is<T>() ? &this->as<T>() : nullptr;
+    }
+
+    /**
+     * @return    The string representation of this value.
+     */
+    SkString toString() const;
+
+protected:
+    /*
+      Value implementation notes:
+
+        -- fixed 64-bit size
+
+        -- 8-byte aligned
+
+        -- union of:
+
+             bool
+             int32
+             float
+             char[8] (short string storage)
+             external payload (tagged) pointer
+
+         -- highest 3 bits reserved for type storage
+
+     */
+    enum class Tag : uint8_t {
+        // We picked kShortString == 0 so that tag 0x00 and stored max_size-size (7-7=0)
+        // conveniently overlap the '\0' terminator, allowing us to store a 7 character
+        // C string inline.
+        kShortString                  = 0b00000000,  // inline payload
+        kNull                         = 0b00100000,  // no payload
+        kBool                         = 0b01000000,  // inline payload
+        kInt                          = 0b01100000,  // inline payload
+        kFloat                        = 0b10000000,  // inline payload
+        kString                       = 0b10100000,  // ptr to external storage
+        kArray                        = 0b11000000,  // ptr to external storage
+        kObject                       = 0b11100000,  // ptr to external storage
+    };
+    static constexpr uint8_t kTagMask = 0b11100000;
+
+    void init_tagged(Tag);
+    void init_tagged_pointer(Tag, void*);
+
+    Tag getTag() const {
+        return static_cast<Tag>(fData8[kTagOffset] & kTagMask);
+    }
+
+    // Access the record data as T.
+    //
+    // This is also used to access the payload for inline records.  Since the record type lives in
+    // the high bits, sizeof(T) must be less than sizeof(Value) when accessing inline payloads.
+    //
+    // E.g.
+    //
+    //   uint8_t
+    //    -----------------------------------------------------------------------
+    //   |  val8  |  val8  |  val8  |  val8  |  val8  |  val8  |  val8  |    TYPE|
+    //    -----------------------------------------------------------------------
+    //
+    //   uint32_t
+    //    -----------------------------------------------------------------------
+    //   |               val32               |          unused          |    TYPE|
+    //    -----------------------------------------------------------------------
+    //
+    //   T* (64b)
+    //    -----------------------------------------------------------------------
+    //   |                        T* (kTypeShift bits)                      |TYPE|
+    //    -----------------------------------------------------------------------
+    //
+    template <typename T>
+    const T* cast() const {
+        static_assert(sizeof (T) <=  sizeof(Value), "");
+        static_assert(alignof(T) <= alignof(Value), "");
+        return reinterpret_cast<const T*>(this);
+    }
+
+    template <typename T>
+    T* cast() { return const_cast<T*>(const_cast<const Value*>(this)->cast<T>()); }
+
+    // Access the pointer payload.
+    template <typename T>
+    const T* ptr() const {
+        static_assert(sizeof(uintptr_t)     == sizeof(Value) ||
+                      sizeof(uintptr_t) * 2 == sizeof(Value), "");
+
+        return (sizeof(uintptr_t) < sizeof(Value))
+            // For 32-bit, pointers are stored unmodified.
+            ? *this->cast<const T*>()
+            // For 64-bit, we use the high bits of the pointer as tag storage.
+            : reinterpret_cast<T*>(*this->cast<uintptr_t>() & kTagPointerMask);
+    }
+
+private:
+    static constexpr size_t kValueSize = 8;
+
+    uint8_t fData8[kValueSize];
+
+#if defined(SK_CPU_LENDIAN)
+    static constexpr size_t kTagOffset = kValueSize - 1;
+
+    static constexpr uintptr_t kTagPointerMask =
+            ~(static_cast<uintptr_t>(kTagMask) << ((sizeof(uintptr_t) - 1) * 8));
+#else
+    // The current value layout assumes LE and will take some tweaking for BE.
+    static_assert(false, "Big-endian builds are not supported at this time.");
+#endif
+};
+
+class NullValue final : public Value {
+public:
+    static constexpr Type kType = Type::kNull;
+
+    NullValue();
+};
+
+class BoolValue final : public Value {
+public:
+    static constexpr Type kType = Type::kBool;
+
+    explicit BoolValue(bool);
+
+    bool operator *() const {
+        SkASSERT(this->getTag() == Tag::kBool);
+        return *this->cast<bool>();
+    }
+};
+
+class NumberValue final : public Value {
+public:
+    static constexpr Type kType = Type::kNumber;
+
+    explicit NumberValue(int32_t);
+    explicit NumberValue(float);
+
+    double operator *() const {
+        SkASSERT(this->getTag() == Tag::kInt ||
+                 this->getTag() == Tag::kFloat);
+
+        return this->getTag() == Tag::kInt
+            ? static_cast<double>(*this->cast<int32_t>())
+            : static_cast<double>(*this->cast<float>());
+    }
+};
+
+template <typename T, Value::Type vtype>
+class VectorValue : public Value {
+public:
+    using ValueT = T;
+    static constexpr Type kType = vtype;
+
+    size_t size() const {
+        SkASSERT(this->getType() == kType);
+        return *this->ptr<size_t>();
+    }
+
+    const T* begin() const {
+        SkASSERT(this->getType() == kType);
+        const auto* size_ptr = this->ptr<size_t>();
+        return reinterpret_cast<const T*>(size_ptr + 1);
+    }
+
+    const T* end() const {
+        SkASSERT(this->getType() == kType);
+        const auto* size_ptr = this->ptr<size_t>();
+        return reinterpret_cast<const T*>(size_ptr + 1) + *size_ptr;
+    }
+
+    const T& operator[](size_t i) const {
+        SkASSERT(this->getType() == kType);
+        SkASSERT(i < this->size());
+
+        return *(this->begin() + i);
+    }
+};
+
+class ArrayValue final : public VectorValue<Value, Value::Type::kArray> {
+public:
+    ArrayValue(const Value* src, size_t size, SkArenaAlloc& alloc);
+};
+
+class StringValue final : public Value {
+public:
+    static constexpr Type kType = Type::kString;
+
+    StringValue();
+    StringValue(const char* src, size_t size, SkArenaAlloc& alloc);
+
+    size_t size() const {
+        switch (this->getTag()) {
+        case Tag::kShortString:
+            // We don't bother storing a length for short strings on the assumption
+            // that strlen is fast in this case.  If this becomes problematic, we
+            // can either go back to storing (7-len) in the tag byte or write a fast
+            // short_strlen.
+            return strlen(this->cast<char>());
+        case Tag::kString:
+            return this->cast<VectorValue<char, Value::Type::kString>>()->size();
+        default:
+            return 0;
+        }
+    }
+
+    const char* begin() const {
+        return this->getTag() == Tag::kShortString
+            ? this->cast<char>()
+            : this->cast<VectorValue<char, Value::Type::kString>>()->begin();
+    }
+
+    const char* end() const {
+        return this->getTag() == Tag::kShortString
+            ? strchr(this->cast<char>(), '\0')
+            : this->cast<VectorValue<char, Value::Type::kString>>()->end();
+    }
+};
+
+struct Member {
+    StringValue fKey;
+          Value fValue;
+};
+
+class ObjectValue final : public VectorValue<Member, Value::Type::kObject> {
+public:
+    ObjectValue(const Member* src, size_t size, SkArenaAlloc& alloc);
+
+    const Value& operator[](const char*) const;
+
+private:
+    // Not particularly interesting - hiding for disambiguation.
+    const Member& operator[](size_t i) const = delete;
+};
+
+class DOM final : public SkNoncopyable {
+public:
+    DOM(const char*, size_t);
+
+    const Value& root() const { return fRoot; }
+
+    void write(SkWStream*) const;
+
+private:
+    SkArenaAlloc fAlloc;
+    Value        fRoot;
+};
+
+inline Value::Type Value::getType() const {
+    switch (this->getTag()) {
+    case Tag::kNull:        return Type::kNull;
+    case Tag::kBool:        return Type::kBool;
+    case Tag::kInt:         return Type::kNumber;
+    case Tag::kFloat:       return Type::kNumber;
+    case Tag::kShortString: return Type::kString;
+    case Tag::kString:      return Type::kString;
+    case Tag::kArray:       return Type::kArray;
+    case Tag::kObject:      return Type::kObject;
+    }
+
+    SkASSERT(false); // unreachable
+    return Type::kNull;
+}
+
+} // namespace skjson
+
+#endif // SkJSON_DEFINED
+